Communication systems based on modulated light sources are well known to the art. In high-speed communication systems, the light source is typically a laser. At frequencies below 10 GHz, the modulation can be imparted to the light source by turning the laser on and off. Unfortunately, this type of laser modulation leads to increased line width in the laser light. At frequencies at or above approximately 10 Ghz, this increased line width cannot be tolerated.
Accordingly, light sources that are to be modulated at frequencies above 10 GHz are typically constructed by providing a laser that runs continuously and a separate light modulator that modulates the intensity of the laser output. Modulators based on electro-absorption utilize a structure that is similar to a laser in that it includes a number of quantum well layers through which the light must propagate. The modulator typically has a transmissive state and an opaque state, which are switched back and forth by applying a potential across the modulator. The electrodes to which the signal is applied present a capacitive load to the driving circuitry, and hence, the modulator section is preferably as short as possible to minimize this capacitive load. In addition, high frequency driving circuitry preferably switches relatively small voltages, since such circuitry utilizes very small transistors that cannot withstand large voltages. Hence, low voltage, short modulators are preferred. Unfortunately, the length of the modulator must be sufficient to provide the desired contrast between the transmissive and opaque states of the modulator.
One promising design that provides short modulator sections that can operate at low voltage utilizes a resonant cavity that is coupled to a waveguide through which the signal that is to be modulated propagates. At “critical coupling”, the losses incurred by the light in making one trip around the resonator exactly equals the amount of light that is coupled into the resonator. When this occurs with light that has a wavelength equal to one of the resonances of the resonator, all of the light in the waveguide is extinguished, and hence the system has a transmission of 0. When the loss around the resonator is not at the critical coupling level, a portion of the light travels down the waveguide.
The amount of light absorbed in the resonator at each pass is determined by a voltage placed across the resonator. The voltage is set such that the resonator is critically coupled at a first voltage and less than critically coupled at a second voltage. Hence, by switching the voltage across the resonator between these two values, the light traveling in the waveguide is modulated from 0 to some transmission T that depends on the losses in the resonator at the second voltage. Ideally, T is equal to 1. That is, all of the light entering the waveguide leaves the waveguide in the transmissive state of the modulator. To achieve this ideal state, all of the losses in the resonator at the second voltage must be zero. This condition is difficult to meet in practical resonators, and hence, modulators of this design are less than ideal.